GAC Adsorber Design for Removal of Chlorinated Pesticides

The cost‐effective removal of alachlor and heptachlor from drinking water by GAC adsorption was investigated. Alachlor and heptachlor represent two classes of chlorinated pesticides: the acetanilides and the cyclodienes, respectively. Transformation of these pesticides prior to and upon adsorption is an important aspect concerning their fate. In aqueous solution, heptachlor undergoes complete hydrolysis to 1‐hydroxychlordene (1‐HC) with a half‐life of three days decreeing the use of 1‐HC in GAC investigation. The dispersed‐flow homogeneous‐surface diffusion model (DFHSDM) was used for predicting the dynamic behavior of the fixed‐bed adsorber for the target pollutants. Model parameters were obtained from bench‐scale equilibrium, rate, and mini‐column experiments. The model was able to successfully predict/simulate mini‐column data in both organic free water and humic acid background solution. Significant reduction in adsorber performance was experienced in the presence of humic acid due to competitive interaction. Fulle‐scale adsorbers were designed through up‐scaling procedures to estimate carbon utilization rates. Cost for removal of alachlor was $0,113/1,000 gal $(\$0.03/m^3)$ treated in a 10‐mgd $(38\times {10}^3{m}^3/d)$ plant with 5‐min EBCT, and 4 gpm/sq ft $(235m^3/m^2/d)$ surface loading for an influent concentration of 100 μg/L and treatment objective of 2 μg/L. The cost was increased to $0.135/1,000 gal in the presence of humic acid. The cost for removal of 1‐HC was, in general, 30% higher than that for alachlor.